Besides graphene, physicists also make efforts to search for other
physical systems, e.g. patterned 2D electron gases [1]} and
ultracold atoms in the honeycomb optical lattice
[2]}, [3]}, [4]}, [5]} , to simulate massless Dirac fermions.
Realization of honeycomb optical lattice opens new possibility of
studying Dirac fermions in cold atoms which provide an extremely
clean environment and controllable fashion unique access to the
study of complex physics [6]}, [7]}, [8]}.
Nevertheless, all of the above systems require the hexagonal
symmetry. Then, it is very attractive to find a system without the
hexagonal symmetry to observe massless Dirac fermions.
